One of the major advances in olefin polymerization technology has been the development of commercially useful metallocene based catalyst systems. Among other advantages, metallocene catalysts allow the production of polyolefins with unique properties such as narrow molecular weight distribution. These properties in turn result in improved structural performance in products made with the polymers such as greater impact strength and clarity in films.
While metallocene catalysts have yielded polymers with improved characteristics, they have presented new challenges when used in traditional polymerization systems. One such area has been in the control of “sheeting” and the related phenomena “drooling” when metallocene catalysts are used in fluidized bed reactors such as those described in U.S. Pat. Nos. 5,436,304 and 5,405,922. “Sheeting” is the adherence of fused catalyst and resin particles to the walls of the reactor. “Drooling” or dome sheeting occurs when sheets of molten polymer form on the reactor walls, usually in the expanded section or “dome” of the reactor, and flow along the walls of the reactor and accumulate at the base of the reactor. Dome sheets are typically formed much higher in the reactor, on the conical section of the dome, or on the hemi-spherical head on the top of the reactor.
Sheeting has been a problem in commercial gas phase polyolefin production reactors for many years. The problem is characterized by the formation of large, solid masses of polymer on the walls of the reactor. These solid masses or polymer (the sheets) eventually become dislodged from the walls and fall into the reaction section, where they interfere with fluidization, block the product discharge port, and usually force a reactor shut-down for cleaning.
Various methods for controlling sheeting have been developed. These often involve monitoring the static charges near the reactor wall in regions where sheeting is known to develop and introducing a static control agent into the reactor when the static levels fall outside a predetermined range. For example, U.S. Pat. Nos. 4,803,251 and 5,391,657 disclose the use of various chemical additives in a fluidized bed reactor to control static charges in the reactor. A positive charge generating additive is used if the static charge is negative, and a negative charge generating additive is used if the static charge is positive. The static charge in the reactor is measured at or near the reactor wall at or below the site where sheet formation usually occurs, using static voltage indicators such as voltage probes or electrodes.
U.S. Pat. Nos. 4,803,251 and 5,391,657 disclose that static plays an important role in the sheeting process with Ziegler-Natta catalysts. When the static charge levels on the catalyst and resin particles exceed certain critical levels, the particles become attached by electrostatic forces to the grounded metal walls of the reactor. If allowed to reside long enough on the wall under a reactive environment, excess temperatures can result in particle fusion and melting, thus producing the sheets or drools.
One of the first descriptions of reactor sheeting was provided in U.S. Pat. No. 4,532,311. The '311 patent also teaches the use of a reactor static probe (the voltage probe) to obtain an indication of the degree of electrification of the fluid bed. U.S. Pat. No. 4,855,370 combined the static probe with addition of water to the reactor (in the amount of 1 to 10 ppm of the ethylene feed) to control the level of static in the reactor. This process has proven effective for Ziegler-Natta catalysts, but has not been effective for metallocene catalysts.
For conventional catalyst systems such as traditional Ziegler-Natta catalysts or chromium-based catalysts, sheet formation usually occurs in the lower part of the fluidized bed. Formation of dome sheets rarely occurs with Ziegler-Natta catalysts. For this reason, the static probes or voltage indicators have traditionally been placed in the lower part on the reactor. For example, in U.S. Pat. No. 5,391,657, the voltage indicator was placed near the reactor distributor plate. See also U.S. Pat. No. 4,855,370. The indicators were also placed close to the reactor wall, normally less than 2 cm from the wall.
U.S. Pat. No. 6,548,610 describes a method of preventing dome sheeting (or “drooling”) by measuring the static charge with a Faraday drum and feeding static control agents to the reactor as required to maintain the measured charge within a predetermined range. U.S. Pat. No. 6,548,610 also discloses that conventional static probes, such as those described in U.S. Pat. Nos. 6,008,662, 5,648,581, and 4,532,311. The method of static measurement described in U.S. Pat. No. 6,548,610 (a Faraday drum) is relatively complicated. Other background references include WO 99/61485, WO 2005/068507, EP 0 811 638 A, EP 1 106 629 A, and U.S. Patent Application Publication No. 2002/103072.
As described above, in contrast to the typical wall sheeting observed for Ziegler-Natta catalysts, use of metallocene catalysts can result in either or both wall sheets and dome sheets. While various methods have been developed to manage the sheeting problems with metallocenes, there has been no root-cause solution because the mechanism of sheeting with metallocene catalyst has not been determined.
One of the most difficult aspects of the sheeting problem with metallocene catalysts has been the lack of advanced warning. Most sheeting incidents with metallocenes have occurred with no advanced indication by any of the known process instruments, including the conventional static probes. This lack of indication with conventional instruments has presented a significant challenge in efforts to troubleshoot and correct the sheeting problems encountered when using metallocenes.
Accordingly, there exists a need for an effective method for determining and controlling the static charge in a fluidized bed reactor, especially for use with metallocene catalyst systems.